1. Field of the Invention
The present invention relates to photolithography, and more specifically to techniques for calibrating models that represent the response of the reticles, particularly to account for the impact of the reticle topography on the electromagnetic field (“EMF”) transmitted through it, known as electromagnetic field (“EMF”) effects thereof.
2. Description of the Related Art
The fabrication of semiconductor chips requires the precise creation of patterns on a semiconductor wafer. Photolithography is a technique used to create patterns on a semiconductor wafer by exposing a layer of photoresist material on the wafer with an image cast by a beam of light transmitted through a reticle, also referred to as a photomask or “mask”. The resist material is a photosensitive polymer that reacts to the light and will develop away in areas with enough light intensity exposure (assuming a positive tone resist). The reticle or mask typically includes a plate of transparent material such as quartz or glass, and a series of opaque metallic features, e.g., chrome features thereon, which correspond to the patterns to be created by the exposure of the photoresist layer.
The resolution of an optical photolithography system is limited by the wavelength of the illumination source, the optical properties of photolithography exposure equipment, or both. In practice, due to the lack of transparent optical components at shorter wavelengths, and the lack of adequate optical sources, sources of wavelength no shorter than 193 nanometer are currently used in advanced photolithography, even as the required minimum feature size on the wafer continues to shrink. This means that lithographic processes are required to print at ever deeper sub-wavelength scales. To help achieve the smallest size patterns in the photoresist (hereinafter “resist”) layer, advances in the resist process and techniques for enhancing resolution such as the deliberate distortion of the mask design to pre-compensate for optical proximity effects (so-called optical proximity corrections or OPC), and masks that include phase shifting features have all been applied in various forms and combinations. The application of OPC algorithms to a full chip consists of the iterative simulation of the predicted printed resist contours and the movement of mask edges until these contours print the desired design on the wafer. In order to accurately predict the contours that will be printed on the resist layer, accurate models of the lithography process are required, including models for the transmission of light through the mask. These models are applied to a full chip design with billions of features. Therefore, the simulation performed for a design must have a manageable runtime, in addition to being accurate.
Another technique of resolution enhancement are phase shifting masks. For example, an alternating phase shift mask can include a transparent substrate, opaque features overlying the transparent substrate, and alternating phase shift features which can be provided in an additional transparent layer of the mask. One example of phase shifting masks is a so-called attenuated phase shifting mask (atten. PSM) which include a transparent substrate, and phase-shifting features overlying the substrate comprised of a material and having a height that limits light transmission through them, such as to a range between 6 and 20% of the light incident thereon. The phase-shifting features are designed to introduce exactly 180 degrees of phase shift relative to the light that propagates through clear openings of the mask which do not have phase-shifting features. These techniques of resolution enhancement have contributed to reduce the minimum feature size printed on a wafer in today's manufacturing processes, while maintaining the same illumination wavelength. In addition, while the features on masks typically have dimensions several, e.g., four times, larger than the features printed on the wafer, even these mask features are now approaching subwavelength dimensions, that is, dimensions smaller than the wavelength of the light.
One requirement for creating and using a mask in advanced sub-wavelength photolithography is to account for the degree to which the size and spacing of the features of the mask can influence the transmission of light through the mask relative to scalar approximations used in practice to model this transmission and that assume perfectly thin mask films. These influences include electromagnetic field (“EMF”) effects which arise as a result of the thicknesses of the substrate, the opaque features thereon, and the phase shift features thereof.
Particularly when photolithography is used to define features smaller than the wavelength of the illumination source, the EMF effects of the mask can shift the locations of the edges of printed features. When patterning such features, the EMF effects need to be accounted for during OPC computations in order to guarantee printing the feature edges in the proper locations. One way that the EMF effects of a mask have been modeled is through numerical computation of Maxwell equations of electromagnetic propagation through the reticle. Computation of Maxwell equations is the most rigorous simulation of the fields transmitted through the mask, which accounts for the mask's optical properties, thicknesses of each opaque feature, the transparent substrate and the phase shift features to arrive at an accurate characterization of the light transmission through the mask that considers EMF effects. However, computing Maxwell equations to calculate the EMF effects through simulation is very computationally intensive.
Still further improvements can be made in methods by which the EMF effects of a mask can be accounted for through the use of simplified models of mask transmission.